Image processing apparatus, control method thereof, and storage medium

ABSTRACT

Medical image data is identified, and a lung field region and an emphysema region in each of a plurality of tomographic images are extracted. A mechanism is provided, which is capable of calculating the ratio of the emphysema region to the lung field region, and displaying an image of the medical image data and a value representing the calculated ratio in association with each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.15/336,238 filed on Oct. 27, 2016 which claims the benefit of JapanesePatent Application No. 2015-213476 filed Oct. 29, 2015 and No.2016-192784 filed Sep. 30, 2016, all of which are hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

This disclosure relates to an image processing apparatus, a controlmethod thereof, and a storage medium.

Description of the Related Art

Emphysema is a type of obstructive pulmonary (lung) disease. Afterprogression of destruction of alveoli mainly caused by cigarettesmoking, emphysema exhibits enlargement of airspaces. The number ofemphysema patients has been on the rise in recent years.

For tests and diagnoses of emphysema, a method called spirometry, whichis for measuring breathing capacity, is mainly used. Spirometry involvesmeasuring the amount of air that is actually inhaled and exhaled. Forpatients with severe emphysema, this measurement may become painful asthe ability to breathe decreases.

Accordingly, quantitative measurements using medical image data (whichmay hereinafter be also referred to as “CT image”) captured by an X-raycomputed tomography (CT) apparatus have been proposed in recent years.“Guidelines for the diagnosis and treatment of COPD (chronic obstructivepulmonary disease), 2nd ed., The Japanese Respiratory Society, MedicalReview Co., Ltd. (2004)” states that a technique called “Goddard method”in which the stages of emphysema progression are expressed in scores isparticularly simple and has been widely used. Therefore, there has beena demand for analysis software that enables evaluation based on theGoddard method.

The evaluation based on the Goddard method uses cross-sections of threelung fields, an upper lung field (near the aortic arch), a middle lungfield (at the bronchial bifurcation), and a lower lung field (1 to 3 cmabove the diaphragm). For each of the right and left lungs, the ratio ofthe area of a low attenuation area (LAA) to the area of each of thethree lung fields is expressed in a score, and the severity of thechronic obstructive pulmonary disease is determined on the basis of thesum of the resulting scores. Note that the LAA is a region having a CTnumber clearly lower than that of a normal lung field region in a CTimage. Typically, a region having a CT number of −950 HU or less isregarded as an LAA.

The measurements made at the six points on the right and left describedabove do not fully determine the condition of the entire lungs that arevertically long organs. Besides the score evaluation at the six pointsdescribed above, it may be necessary to provide a mechanism that allowsa diagnostician to visually and intuitively determine how the conditiondevelops in the body axis direction.

SUMMARY OF THE INVENTION

Accordingly, this disclosure provides a mechanism that allows a user torecognize the location where there are many lesions in the entire lungfields.

An image processing apparatus according to the disclosure includes anacquiring unit configured to acquire medical image data of a subject; afirst extracting unit configured to extract a lung field region in eachof a plurality of tomographic images of the medical image data acquiredby the acquiring unit; a second extracting unit configured to extract anemphysema region in the lung field region extracted by the firstextracting unit; a calculating unit configured to calculate, for each ofthe plurality of tomographic images, a ratio of the emphysema region tothe lung field region; and a display control unit configured to performcontrol to display, in a display unit, an image of the medical imagedata and a value representing the ratio of the emphysema regioncalculated by the calculating unit such that the value representing theratio of the emphysema region and a position of the tomographic imagecorresponding to the value are identifiable.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a hardware configuration of a medical imageprocessing apparatus according to an embodiment of the presentinvention.

FIG. 2 illustrates a functional configuration of the medical imageprocessing apparatus according to an embodiment of the presentinvention.

FIG. 3 is a flowchart illustrating a flow of processing according to anembodiment of the present invention.

FIG. 4 shows a screen in which an image of medical image data and agraph are displayed side by side according to an embodiment of thepresent invention.

FIG. 5 shows a screen in which an image of medical image data and graphsfor right and left lungs are displayed side by side according to anembodiment of the present invention.

FIG. 6 shows a screen in which an image of medical image data and agraph, which is changed in display size in accordance with a change inthe display size of the image, are displayed side by side according toan embodiment of the present invention.

FIG. 7 shows a screen in which an image of medical image data and agraph, which is changed in display position in accordance with movementof the image, are displayed side by side according to an embodiment ofthe present invention.

FIG. 8 shows a screen in which an image of medical image data and agraph for both the right and left lungs are displayed according to athird embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will now be described withreference to the drawings.

With reference to FIG. 1, a hardware configuration of a medical imageprocessing apparatus (which may hereinafter be simply referred to as“image processing apparatus”) 100 according to an embodiment of thepresent invention will be described. Note that the hardwareconfiguration of the image processing apparatus 100 illustrated in FIG.1 is merely an example, and various other configurations are possibledepending on the application and purpose of use.

The image processing apparatus 100 includes a central processing unit(CPU) 201, a random-access memory (RAM) 202, a read-only memory (ROM)203, a system bus 204, an input controller 205, a video controller 206,a memory controller 207, a communication interface (I/F) controller 208,an input device 209, a display 210, and an external memory 211.

The CPU 201 provides comprehensive control over devices and controllersconnected to the system bus 204.

The RAM 202 serves as a main memory and a work area for the CPU 201. TheCPU 201 loads a program required for execution of processing into theRAM 202 and executes the program, thereby implementing variousoperations.

The ROM 203 or the external memory 211 stores a basic input/outputsystem (IOS), which is a control program for the CPU 201, an operatingsystem (OS), and various programs required for implementing functionsexecuted by various devices.

The input controller 205 controls an input from the input device 209,such as a keyboard or a pointing device (e.g., mouse).

The video controller 206 controls the display of a display device, suchas the display 210. Examples of the display 210 (display unit) include acathode-ray tube (CRT) and a liquid crystal display.

The memory controller 207 controls access to the external memory 211,such as a hard disk drive (HDD) or flexible disk that stores a bootprogram, browser software, various applications, font data, user files,and various types of data, or a card-type memory connected to a PersonalComputer Memory Card International Association (PCMCIA) card slot via anadapter.

The CPU 201 enables display on the display 210, for example, byexpanding (rasterizing) an outline font in a display information regionin the RAM 202. Also, the CPU 201 allows a user to give instructionswith a mouse cursor (not shown) on the display 210.

Various programs used for the image processing apparatus 100 to executevarious types of processing (described below) are stored in the externalmemory 211. The programs are loaded into the RAM 202 as necessary, andexecuted by the CPU 201. Definition files, various information tables,and medical images used by a program according to an embodiment of thepresent invention are stored in the external memory 211. The medicalimages may be stored, for example, on an external server and acquiredtherefrom by the image processing apparatus 100.

The description of the hardware configuration of the image processingapparatus 100 illustrated in FIG. 1 ends here.

A functional configuration of the image processing apparatus 100 willnow be described with reference to FIG. 2.

The image processing apparatus 100 includes, as functional units, animage acquiring unit 1001, a lung field region extracting unit 1002, anemphysema region extracting unit 1003, a calculating unit 1004, a graphgenerating unit 1005, and a display control unit 1006. The imageacquiring unit 1001 acquires medical image data, such as CT images of asubject captured by a modality. The lung field region extracting unit1002 extracts a lung field region of the subject on the basis of themedical image data acquired by the image acquiring unit 1001. Theemphysema region extracting unit 1003 extracts an emphysema region ofthe subject on the basis of the medical image data acquired by the imageacquiring unit 1001. The calculating unit 1004 calculates, for eachmedical image data, the ratio of the extracted emphysema region to theextracted lung field region. The graph generating unit 1005 generates agraph that shows the ratio of the emphysema region to the lung fieldregion calculated by the calculating unit 1004 and a subject's positionin the medical image data corresponding to the ratio. The displaycontrol unit 1006 causes an image of the subject and the graph generatedby the graph generating unit 1005 to be displayed on a display screen.

The description of the functional configuration of the image processingapparatus 100 illustrated in FIG. 2 ends here.

Processing according to embodiments of the present invention will now bedescribed in detail with reference to the flowchart of FIG. 3.

First Embodiment

In step S301, the CPU 201 of the image processing apparatus 100acquires, for example, a group of CT images, which are sliced images(tomographic images) captured by an X-ray CT apparatus. In the field ofmedicine, images captured by such a modality device are typicallytransmitted to a picture archiving communication system (PACS) connectedto the modality device via a network, and are centrally managed by thePACS. Accordingly, the image processing apparatus 100 is configured suchthat the images can be acquired not only internally, but also from thePACS. The CPU 201 of the image processing apparatus 100 may acquire notonly sliced images, but also resliced images obtained by reslicing athree-dimensional image formed by a plurality of sliced images.

In step S302, the CPU 201 of the image processing apparatus 100 replacesthe CT number of a background portion, which is not a human body regionin each of the acquired sliced images, with a CT number greater thanthose corresponding to a lung field region. This is to prevent thebackground portion from being accidentally extracted during extractionof an emphysema region later on. Specifically, the CT number of thebackground portion is replaced with −700 HU.

Then, a lung field region is extracted for each sliced image byidentifying pixels with CT numbers smaller than a predeterminedthreshold (e.g., CT number: −400 HU), which is a reference value forextracting a lung field region. The lung field region extracting methodis not limited to this. That is, as long as a lung field region can beidentified, user's designation of the lung field region may be receivedor any other method, such as that described in Japanese Patent Laid-OpenNo. 2003-70781, may be used.

In step S303, the CPU 201 of the image processing apparatus 100 extractsan emphysema region for each of the sliced images acquired in step S301.An emphysematous lesion is extracted as an LAA in the sliced image. Morespecifically, the emphysema region is extracted by identifying pixelswith CT numbers smaller than a known value (e.g., CT number: −900 HU)corresponding to emphysema. The emphysema region extracting method isnot limited to this. Any other method, such as that described inJapanese Patent Laid-Open No. 2003-70781, may be used to extract theemphysema region.

In step S304, the CPU 201 of the image processing apparatus 100calculates, for each sliced image, the ratio of the emphysema regionextracted in step S303 to the lung field region extracted in step S302.More specifically, the CPU 201 calculates the ratio by dividing N by M,where M is the number of pixels in the lung field region and N is thenumber of pixels in the emphysema region. Thus, for each sliced image,the CPU 201 calculates a value representing the ratio of the emphysemaregion to the lung field region. The calculated value is managed inassociation with the sliced image.

In step S305, the CPU 201 of the image processing apparatus 100generates a graph by plotting the value representing the ratiocalculated for each sliced image in step S304. The generated graph is agraph 401 shown in FIG. 4. The graph 401 in FIG. 4 has reference linescorresponding to 25%, 50%, and 75%, which are reference values used forscore determination in the Goddard method and each representing theratio of emphysema. In the graph 401 in FIG. 4, a curve segment withinthe range of emphysema ratios from 0% to less than 25% is shown in blue,a curve segment within the range of emphysema ratios from 25% to lessthan 50% is shown in orange, a curve segment within the range ofemphysema ratios from 50% to less than 75% is shown in white, and acurve segment within the range of emphysema ratios from 75% to less than100% is shown in red. Thus, by varying the display of the graph stepwisein accordance with the value representing the emphysema ratio, the usercan clearly recognize which part of the entire lung fields of thesubject is significantly affected by emphysema.

In step S306, the CPU 201 of the image processing apparatus 100displays, side by side, a coronal cross-sectional image generated byreslicing a three-dimensional image formed by the group of sliced imagesacquired in step S301 and the graph generated in step S305. As shown inFIG. 4, by displaying a coronal cross-sectional image 402 and the graph401 side by side on a screen, the user can easily compare a valuerepresenting an emphysema ratio with a position in the subjectcorresponding to the value. That is, the user can identify whichposition in the subject corresponds to a portion indicating a highemphysema ratio in the graph, and can recognize at a glance thedistribution of emphysema which cannot be known only from scorescalculated on the basis of the Goddard method.

In step S307, the CPU 201 of the image processing apparatus 100determines whether a user's selecting operation with the input device209 (e.g., mouse click) has been received on the graph 401 displayed instep S306. If the CPU 201 determines that the selecting operation hasbeen received, the process proceeds to step S308, and otherwise theprocess proceeds to step S310.

In step S308, the CPU 201 of the image processing apparatus 100identifies a position on the graph 401 at which the selecting operationhas been received in step S307, and then identifies a sliced imagecorresponding to a value representing the ratio at the identifiedposition.

In step S309, the CPU 201 of the image processing apparatus 100displays, in a display region 403 (see FIG. 4), the sliced imageidentified in step S308. Thus, when the user wants to refer to a slicedimage corresponding to a value on the graph 401, the sliced image can beeasily displayed on the display screen. After the sliced image isdisplayed in the display region 403, the process returns to step S307.

In step S310, the CPU 201 of the image processing apparatus 100determines whether an instruction to enlarge or reduce (i.e., change thedisplay magnification of) the coronal cross-sectional image 402 and aninstruction to move the display position of the coronal cross-sectionalimage 402 have been received from the user. If the instruction toenlarge or reduce and the instruction to move the display position havebeen received, the process proceeds to step S311, and otherwise theprocess proceeds to step S312.

In step S311, the CPU 201 of the image processing apparatus 100 changesthe display size and the display position of the coronal cross-sectionalimage 402 in accordance with the user's instructions received in stepS310. Then, while maintaining the display size and position of thecoronal cross-sectional image 402 changed in accordance with the user'sinstructions (or in synchronization with changes in the display size andposition of the coronal cross-sectional image 402), the CPU 201 changesthe display size and position of the graph 401 showing the emphysemaratio.

An example will be described with reference to FIG. 6. FIG. 6 shows ascreen in which the size of the coronal cross-sectional image 402 inFIG. 4 has been changed. The display size of a coronal cross-sectionalimage 602 is reduced in accordance with an instruction from the user.The display size of a graph 601 is reduced to the scale of the coronalcross-sectional image 602, with the positional relationship between thegraph 601 and the coronal cross-sectional image 602 maintained. Althoughthe coronal cross-sectional image 602 is displayed in a reduced size inthe present embodiment, it may of course be displayed in an enlargedsize.

FIG. 7 shows a screen in which the position of the coronalcross-sectional image 402 in FIG. 4 has been changed. The displayposition of a graph 701 is changed in synchronization with the movementof a coronal cross-sectional image 702, with the positional relationshipbetween the graph 701 and the coronal cross-sectional image 702maintained.

As described above, it is possible not only to change the display sizeand position of the coronal cross-sectional image, but also to changethe display size and position of the graph in synchronization with thecoronal cross-sectional image. This allows the user to view thedistribution of emphysema in the entire lungs, and to easily understandin detail which part of the lungs is significantly affected byemphysema. In the description above, the display size and position ofthe coronal cross-sectional image 402 are changed in accordance with theuser's instructions received in step S310. Alternatively, the displaysize and position of the coronal cross-sectional image 402 may bechanged in accordance with changes in the display size and position ofthe graph 401 made in response to receipt of user's instructions.

In step S312, the CPU 201 of the image processing apparatus 100determines whether an end instruction to end the display has beenreceived from the user. If the CPU 201 determines that the endinstruction has been received, the display process ends, and otherwisethe process returns to step S307.

The description of the flowchart of FIG. 3 ends here.

Second Embodiment

In the first embodiment described above, the lung field region in eachsliced image is not divided into right and left lungs, and values eachrepresenting the ratio of an emphysema region to the entire lung fieldregion are calculated to generate a graph. In a second embodiment, thelung field region is divided into a right lung field region and a leftlung field region. Then, a graph showing values each representing theratio of a right emphysema region to the right lung field region, and agraph showing values each representing the ratio of a left emphysemaregion to the left lung field region are generated. The same parts asthose of the first embodiment will not be described here, anddifferences from the first embodiment will be described.

With reference to the flowchart of FIG. 3, only operations differentfrom those in the first embodiment will be described. Operations notdescribed here are performed in the same manner as in the firstembodiment.

In step S302, the CPU 201 of the image processing apparatus 100 replacesthe CT number of a background portion, which is not a human body regionin each of the acquired sliced images, with a CT number greater thanthose corresponding to a lung field region. This is to prevent thebackground portion from being accidentally extracted during extractionof an emphysema region later on. Specifically, the CT number of thebackground portion is replaced with 800 HU.

Then, a lung field region is extracted for each sliced image byidentifying pixels with CT numbers smaller than a predeterminedthreshold (e.g., CT number: −400 HU), which is a reference value forextracting a lung field region. As shown in a display region 503 in FIG.5, the extracted lung field region includes two regions on the right andleft. These regions may be stored as a right lung field region and aleft lung field region by receiving user's selections with the mouse, ormay be identified as a right lung field region and a left lung fieldregion using a known technique.

In step S303, the CPU 201 of the image processing apparatus 100 extractsa right emphysema region and a left emphysema region in the right lungfield region and the left lung field region, respectively, extracted instep S302. The emphysema region extracting method is the same as that inthe first embodiment. By carrying out the emphysema region extractingmethod within the regions extracted in step S302, the right and leftemphysema regions in the right and left lung field regions areextracted.

In step S304, the CPU 201 of the image processing apparatus 100calculates the ratios of the right and left emphysema regions extractedin step S303 to the respective right and left lung field regions.

In step S305, the CPU 201 of the image processing apparatus 100generates a graph 501 showing values each representing the ratio of theright emphysema region to the right lung field region, and a graph 502showing values each representing the ratio of the left emphysema regionto the left lung field region, as shown in FIG. 5.

In step S306, the CPU 201 of the image processing apparatus 100 displaysthe graphs generated in step S305 at the corresponding positions withrespect to a coronal cross-sectional image. More specifically, the graph501 showing values each representing the ratio of the right emphysemaregion to the right lung field region is displayed adjacent to the rightlung field region in a coronal cross-sectional image 504, and the graph502 showing values each representing the ratio of the left emphysemaregion to the left lung field region is displayed adjacent to the leftlung field region in the coronal cross-sectional image 504. This allowsthe user to recognize how the emphysema progresses in each of the rightlung field region and the left lung field region in the coronalcross-sectional image 504.

The description of the second embodiment ends here.

Third Embodiment

A third embodiment of the present invention will now be described. Inthe second embodiment, a lung field region is divided into a right lungfield region and a left lung field region. Then, a graph showing valueseach representing the ratio of a right emphysema region to the rightlung field region, and a graph showing values each representing theratio of a left emphysema region to the left lung field region aregenerated and displayed separately as in FIG. 5. In the thirdembodiment, the graphs, each showing the ratio of the emphysema regionto the corresponding lung field region, are displayed in the same regionin a superimposed manner as in FIG. 8. The system configuration,hardware configuration, functional configuration, and data tables of thethird embodiment are the same as those of the second embodiment. Thesame operations as those of the second embodiment will not be describedhere, and only operations different from those of the second embodimentwill be described.

With reference to the flowchart of FIG. 3, a flow of processingaccording to the third embodiment will be described in detail. Thedescription of steps S301 to S304 will be omitted, as it is the same asthat in the second embodiment.

In step S305, the CPU 201 of the image processing apparatus 100generates the graph 501 showing values each representing the ratio ofthe right emphysema region to the right lung field region, and the graph502 showing values each representing the ratio of the left emphysemaregion to the left lung field region, as shown in FIG. 8. The graph 501and the graph 502 are identifiable, because the graph 501 and the graph502 are represented by a solid line and a dotted line, respectively, inthe present embodiment. Alternatively, the graph 501 and the graph 502may be displayed in different colors for easy identification.

In step S306, the CPU 201 of the image processing apparatus 100 displaysthe graph 501 and the graph 502 generated in step S305 such that theypositionally correspond to the coronal cross-sectional image 504. In thepresent embodiment, the graph 501 and the graph 502 are displayed in thesame region. A more specific description will be given with reference toFIG. 8.

FIG. 8 shows a screen in which the graph 501 showing values eachrepresenting the ratio of the right emphysema region to the right lungfield region, and the graph 502 showing values each representing theratio of the left emphysema region to the left lung field region, aredisplayed in the same region. As shown, the graph 501 showing the ratioof the right emphysema region to the right lung field region and thegraph 502 showing the ratio of the left emphysema region to the leftlung field region are displayed in the same region in a superimposedmanner. This facilitates comparison of the progression of emphysemabetween the right lung field region and the left lung field region. Thismeans that the user can intuitively recognize which of the right andleft lungs is worse in condition.

The description of the third embodiment ends here.

According to the embodiments of the present invention, the user canunderstand at a glance the distribution of emphysema (LAA) in the entirelung fields, which cannot be known from scores based on the Goddardmethod. It is thus possible to allow the user to recognize the degree ofprogression of subject's symptoms.

In the embodiments of the present invention, a value obtained, for everysliced image forming volume data of the subject captured by the CTapparatus, by calculating the ratio of the emphysema region to the lungfield region is used to generate a graph, which is then displayed sideby side with an image of the subject. However, the calculation does notnecessarily need to be performed for every sliced image forming thevolume data. For example, a value obtained once every several slicedimages by calculating the ratio of the emphysema region to the lungfield region may be used to generate a graph, which is then displayedside by side with an image of the subject.

The present invention may be embodied, for example, as a system, anapparatus, a method, a program, or a storage medium. Specifically, thepresent invention may be applied to a system composed of a plurality ofdevices, or to an apparatus composed of a single device. The presentinvention includes the case of directly or remotely supplying a softwareprogram that implements the functions of the above-described embodimentsto a system or apparatus. The present invention also includes the caseof being achieved when an information processor of the system orapparatus reads out and executes the supplied program code.

Therefore, the program code installed in the information processor forimplementing functional processing of the present invention in theinformation processor also implements the present invention. That is,the present invention also includes a computer program for implementingthe functional processing of the present invention.

In this case, the computer program may be object code, a programexecuted by an interpreter, or script data supplied to an OS, or may beof any other form as long as it has the functions of the program.

Examples of a recording medium for supplying the program include aflexible disk, a hard disk, an optical disc, a magneto-optical disc(MO), a compact disc ROM (CD-ROM), a CD recordable (CD-R), a CDrewritable (CD-RW), a magnetic tape, a nonvolatile memory card, a ROM,and a digital versatile disc (DVD) (DVD-ROM, DVD-R).

The program may be supplied by connecting to a website on the Internetusing a browser on a client computer, and then downloading the computerprogram of the present invention or a compressed file having anautomatic installation function, from the website onto a recordingmedium, such as a hard disk.

The present invention may also be implemented by dividing the programcode forming the program of the present invention into multiple files,and downloading the multiple files from different websites. That is, thepresent invention also includes a World Wide Web (WWW) server thatallows multiple users to download program files for implementing thefunctional processing of the present invention in the informationprocessor.

The program according to the present invention may be encrypted andstored on a storage medium, such as a CD-ROM, and distributed to users.In this case, users satisfying predetermined conditions are permitted todownload key information for decrypting the encrypted program from awebsite via the Internet. By using the downloaded key information, theencrypted program can be executed, installed onto the informationprocessor, and implemented.

The functions of the embodiments described above are implemented whenthe information processor executes a read-out program. The functions ofthe embodiments described above may also be implemented when an OS orthe like running on the information processor performs part or all ofactual processing on the basis of instructions of the program.

The functions of the embodiments described above may also be implementedwhen the program read out from a recording medium is written to a memoryof a function expansion board inserted in the information processor or afunction expansion unit connected to the information processor, and thena CPU or the like of the function expansion board or the functionexpansion unit performs part or all of actual processing on the basis ofinstructions of the program.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. An image processing apparatus comprising: amemory storing a program; and one or more processors which, by executingthe program, function as: an acquiring unit configured to acquiremedical image data of a subject; a first extracting unit configured toextract a lung field region in a plurality of tomographic images of themedical image data acquired by the acquiring unit; a second extractingunit configured to extract an emphysema region in the lung field regionextracted by the first extracting unit; a calculating unit configured tocalculate a ratio of the emphysema region to the lung field region; anda display control unit configured to display, on a display unit, across-sectional image based on the medical image data, wherein thedisplay control unit performs control to display a graph, whichindicates a value that shows the ratio of the emphysema region to thelung field region for each position in a predetermined direction of thesubject, and the cross-sectional image side by side on the display unitsuch that a position in the graph in a body axis direction and aposition in the cross-sectional image in the body axis direction are inalignment.
 2. The image processing apparatus according to claim 1,wherein the display control unit displays the graph at a correspondingposition of the cross-sectional image.
 3. The image processing apparatusaccording to claim 1, wherein the display control unit performs controlsuch that the graph is displayed differently depending on the valuerepresenting the ratio of the emphysema region to the lung field region.4. The image processing apparatus according to claim 1, wherein thecross-sectional image displayed in the display unit is a coronalcross-sectional image.
 5. The image processing apparatus according toclaim 1, wherein the display control unit performs control such that thevalue representing the ratio of the emphysema region to the lung fieldregion and the position of the tomographic image corresponding to thevalue are displayed side by side in the display unit, whereby thecorrespondence therebetween is identifiably displayed.
 6. The imageprocessing apparatus according to claim 1, further comprising a displayposition receiving unit configured to receive a change in a displayposition of the image of the medical image data displayed in the displayunit, wherein the display control unit controls display such that thedisplay position of the image of the medical image data is changed inaccordance with the change received by the display position receivingunit, and that a position for displaying the value representing theratio is also changed to maintain a correspondence with the image of themedical image data.
 7. The image processing apparatus according to claim1, further comprising a display magnification receiving unit configuredto receive a change in a display magnification of the image of themedical image data displayed in the display unit, wherein the displaycontrol unit controls display such that the display magnification of theimage of the medical image data is changed in accordance with the changereceived by the display magnification receiving unit, and that amagnification for displaying the value representing the ratio is alsochanged to maintain a correspondence with the image of the medical imagedata.
 8. The image processing apparatus according to claim 1, whereinthe display control unit displays, side by side, the graph and thecross-sectional image.
 9. The image processing apparatus according toclaim 1, wherein the display control unit displays a line on the graphand the cross-sectional image and the value representing the ratio ofthe emphysema region to the lung field region corresponding to the line.10. The image processing apparatus according to claim 1, wherein thedisplay control unit displays the tomographic image corresponding to thevalue representing the ratio of the emphysema region to the lung fieldregion.
 11. The image processing apparatus according to claim 1, whereinthe display control unit displays a graph showing values eachrepresenting the ratio of the emphysema region to the lung field region.12. The image processing apparatus according to claim 1, wherein thecalculating unit calculates, for each of the plurality of tomographicimages, the ratio of the emphysema region to the lung field region. 13.The image processing apparatus according to claim 1, wherein the firstextracting unit and the second extracting unit extract a left lung fieldregion and a right lung field region, respectively, of the subject; thecalculating unit calculates a ratio of a left emphysema region to theleft lung field region extracted by the first extracting unit, and aratio of a right emphysema region to the right lung field regionextracted by the second extracting unit; and the display control unitperforms control such that a value representing the ratio of the leftemphysema region and a value representing the ratio of the rightemphysema region are separately displayed.
 14. The image processingapparatus according to claim 13, wherein the display control unitdisplays a first graph showing values each representing the ratio of theright emphysema region to the right lung field region, and a secondgraph showing values each representing the ratio of the left emphysemaregion to the left lung field region.
 15. An image processing apparatuscomprising: a memory storing a program; and one or more processorswhich, by executing the program, function as: an acquiring unitconfigured to acquire medical image data of a subject; a firstextracting unit configured to extract a lung field region in a pluralityof tomographic images of the medical image data acquired by theacquiring unit; a second extracting unit configured to extract anemphysema region in the lung field region extracted by the firstextracting unit; a calculating unit configured to calculate a ratio ofthe emphysema region to the lung field region; and a display controlunit configured to display, on a display unit, a cross-sectional imagebased on the medical image data, wherein the display control unitdisplays a graph, which is generated using a value representing theratio of the emphysema region to the lung field region, corresponding tothe cross-sectional image, and wherein the display control unit changesat least one of a display size and a position of the graph insynchronization with the cross-sectional image, in a case where thedisplay control unit changes at least one of a display size and aposition of the cross-sectional image.
 16. The image processingapparatus according to claim 15, wherein the display size of the graphis scaled to the display size of the cross-sectional image, with apositional relationship between the graph and the cross-sectional imagemaintained.
 17. The image processing apparatus according to claim 15,wherein the position of the graph is changed in synchronization withmovement of the cross-sectional image, with a positional relationshipbetween the graph and the cross-sectional image maintained.
 18. Acontrol method of an image processing apparatus, the control methodcomprising: an acquiring step of acquiring medical image data of asubject; a first extracting step of extracting a lung field region ineach of a plurality of tomographic images of the medical image dataacquired in the acquiring step; a second extracting step of extractingan emphysema region in the lung field region extracted in the firstextracting step; a calculating step of calculating a ratio of theemphysema region to the lung field region; and a display control step ofdisplaying, on a display unit, a graph, which indicates a value thatshows the ratio of the emphysema region to the lung field region foreach position in a predetermined direction of the subject, and thecross-sectional image side by side on the display unit such that aposition in the graph in a body axis direction and a position in thecross-sectional image in the body axis direction are in alignment.
 19. Anon-transitory storage medium storing a program that, when executed byone or more processors, causes an image processing apparatus to functionas: an acquiring unit configured to acquire medical image data of asubject; a first extracting unit configured to extract a lung fieldregion in a plurality of tomographic images of the medical image dataacquired by the acquiring unit; a second extracting unit configured toextract an emphysema region in the lung field region extracted by thefirst extracting unit; a calculating unit configured to calculate aratio of the emphysema region to the lung field region; and a displaycontrol unit configured to display, on a display unit, a cross-sectionalimage based on the medical image data, wherein the display control unitperforms control to display a graph, which indicates a value that showsthe ratio of the emphysema region to the lung field region for eachposition in a predetermined direction of the subject, and thecross-sectional image side by side on the display unit such that aposition in the graph in a body axis direction and a position in thecross-sectional image in the body axis direction are in alignment.